Single-phase permanent-magnet synchronous electric motors are constituted by a stator, in which the electromagnet is formed by a lamination pack on which the corresponding windings are wound, and a rotor, which is arranged between the pole shoes formed by the stator.
As is known, these electric motors are of the bidirectional type: in practice, at power-on the rotor can be induced equally to rotate clockwise or counterclockwise; this occurs mainly because the magnetic field generated between the pole shoes of the stator when the induction windings are powered with alternating current is a pulsed field.
In view of these considerations, these synchronous permanent-magnet motors are currently widely used in applications in which the direction of rotation is not important, such as for example in association with impellers with radial vanes of centrifugal pumps, which ensure the same performance in both directions of rotation.
If one wishes to increase the efficiency of pumps driven by a synchronous motor, it is necessary to use impellers with vanes which are appropriately contoured.
To do this, however, it is necessary to control the direction of rotation when the motor is driven; for this reason, electronic starters which allow to control the direction of rotation of rotors of synchronous motors and devices for locking the rotor if it starts in the wrong direction have been studied, allowing in practice to control the direction of rotation in any operating condition of the motor.
One particular solution to allow to start the motor according to a direction of rotation chosen at will is disclosed in Italian Patent No. 1324617 filed by this same Applicant.
This solution describes a synchronous permanent-magnet motor which comprises a stator, with an electromagnet formed by a lamination pack and corresponding windings, and a rotor which is arranged between pole shoes formed by the stator.
The motor is characterized in that it comprises two pairs of pole shoes, each formed by a separate C-shaped lamination pack, which are mutually angularly offset.
The lamination packs support respective adjacent spools with windings.
The windings are for example connected to an AC power supply by using a capacitor and a switch; the position of the switch determines the direction of rotation of the rotor, arranging each winding alternately in series to the capacitor and in parallel to the other winding.
As an alternative, the windings are connected in series to respective triacs, which are arranged between said windings and control electronics for sending sequences of half-waves, or there can be an electronic control with phase slicing of one of the two windings in order to generate a field with a rotating component which, by reversing the actuation of the windings, achieves control over the direction of rotation of the rotor.
The particular C-shaped structure allows to have a motor which is compact and has limited space occupation.
The particular structure of the C-shaped lamination packs is such that the pole shoes of one lamination pack intersect, at a different axial height, the pole shoes of the other lamination pack; the two pairs of pole shoes wrap around the rotor at different axial heights.
This causes the rotor to tend to oscillate axially, if left free, or in any case to vibrate axially, since the electromagnetic flux generated by the pole shoes is alternately at different axial heights (and the rotor tends to be arranged symmetrically with respect to the magnetic flux).